UC Irvine

The net climate effect of aerosols on Earth’s atmosphere is highly uncertain. Secondary organic aerosols (SOA) heavily contribute to this uncertainty because they are chemically complex, and their chemical and physical properties have not been fully characterized. Various atmospheric chemical aging processes further complicate the interpretation of the climate effects of aerosols. The influence of the condensed-phase aging processes, which occur inside aerosol particles is an active, rapidly growing topic of research. Recently, a number of aerosol aging experiments have been discussed; however, a systematic evaluation of the role of the condensed-phase photochemical processes in aerosols has not been performed.

In this work, SOA was generated in a smog chamber or flow reactor from a variety of precursor types (anthropogenic and biogenic) and oxidant conditions. Additionally, experiments were performed on trees. Chapter 2 of this dissertation discusses the condensed-phase photodegradation experiments of the SOA material. The photodegradation is found to be efficient and atmospherically relevant. We detected a variety of volatile organic compounds (VOCs) that were produced during ultraviolet irradiation of SOA via proton transfer reaction time-of-flight mass spectrometry (PTR-ToF-MS). Next, in Chapter 3, we quantify the rate of photodegradation of SOA material with a quartz crystal microbalance (QCM) that is connected in series with the PTR-ToF-MS. Additionally, we discuss that while the mass degradation is atmospherically relevant (1% per hour during the Los Angeles summer solstice), the mass loss is not likely to be a concern for oxidation flow reactors (OFRs) commonly used in atmospheric chemistry experiments.

Chapter 4 probes the possibility of photosensitized SOA reactions where a VOC is taken up into the condensed-phase aerosol material. While this reaction does occur, it is unlikely to compete with faster gas-phase reactions of VOCs. Finally, Chapter 5 explores preliminary experiments focusing on photochemical reactions on Scots pine tree surfaces. A given tree is isolated and coated with SOA generated by ozonolysis of its VOCs, and the tree is subjected to ultraviolet light to probe the photochemical reactions from the SOA coated tree. The results of Chapter 5 are not conclusive, but the photochemical reactions could still be important in the atmosphere.